The incidence and clinical characteristics of mycobacteriosis in patients attending the nephrology department in a hospital in Madrid, Spain, during a 30-month period were analysed retrospectively. Twenty-two new cases of tuberculosis were detected among 525 patients studied. No cases of clinically significant atypical mycobacteriosis were found. The estimated overall annual incidence of tuberculosis in these patients was 259 cases per 100,000 population, which is much higher than the national annual incidence in Spain of 35 per 100,000 population. Most cases were asymptomatic when diagnosed and extrapulmonary involvement was the rule (86% of all patients). In areas with a high prevalence of tuberculosis, renal patients in high-risk groups (renal transplant recipients, haemodialysis and CAPD patients) should be examined periodically to exclude silent infection.